Why I recommend Multi-Zone Controls for Radiant Panel Heating.
Radiant Panel heating systems are generally promoted as being more "comfortable" and "efficient" than other forms of heat distribution. The truth be known, however, is that they can be quite comfortable and efficient, but not, necessarily, because they are radiant floors ceilings or walls. The reason they can be comfortable and efficient is because, by their very nature, they can be constructed in such a manner as to place the heat energy exactly where it is needed in the room, and exactly when it is needed there. That is the essence of comfort and efficiency.
I do a lot of heat loss analysis and performance planning calculations on projects. As I do, I notice that that each room tends to behave differently from others in the building. It is not at all unusual to see a structure with widely varying heat demand between the rooms due to big differences in the size or number of windows, and the ratio of floor area to window area. This, along with variations in floor coverings often results in calculated design supply water temperature differences of 30 or 40 degrees between different rooms in the same structure. Widely varying supply water temperature presents a serious control problem. Notionally, a lot of designers reason that you can adjust the flow for the separate loops and "balance" out these differences. This is true, to a degree, but such control logic easily proves to be unsatisfactory. For example, the major heat loss components for most rooms are usually the windows. Our calculations for the heat loss through windows are totally wrong when the draperies are closed. When the windows are covered, the heat demand for that room is significantly lowered and that changes the whole game plan.
When we perform a heat loss analysis, we are only looking for envelope heat losses. That is, we calculate the amount of energy that will be lost through the various outside surfaces and estimate the amount of energy lost through air exchanges with outside air. That's only a small part of the control equation, however. There are a lot more factors than envelope heat loss that need to be accommodated in the control strategy. We need to consider other forms of heat loss and heat gain.
One of the biggest factors of heat gain is solar gains from direct sunlight through glass and the resultant "greenhouse" affect. Direct sunlight can add 65 BTU/hr per square foot of south facing glass. That is a very large gain which affects only some of the areas of the structure and only at certain times when the sun angle is right. Adding to that complexity, solar heat gain follows a seasonal as well as a daily march from room to room.
There are other significant heat gains that occur on a random basis throughout the structure. Cooking, Bathing, Washing Clothes, and operating large TVs, computers and appliances all provide a huge amount of internal heat gain. You can actually calculate these gains by looking at your electric bill. Since the majority of the energy is consumed within the walls of the house, you can multiply the kilowatts by 3413 per kilowatt to get the Btu internal gains. Gas appliances, such as ranges, stoves and clothes dryers also provide a fair amount of internal gains while they are being used.
Finally, another major internal heat gain factor is occupancy. Each person in the room gives off about 650 Btu's per hour depending on his activity level. One room may have a high occupancy rate, whereas other rooms might not, thereby resulting in a huge disparity in relative heat loads.
Internal losses are not that common in residential buildings, but for factories and warehouses that accept large quantities of cold freight, for example, the internal losses can be many times the envelope loss. A 40,000 pound shipment of cold steel on the warehouse floor can suck a lot of heat.
I would venture, that for some of my customers, probably the largest internal heat loss that occurs in residential buildings is when a Mother-in-law comes to visit.
My point is that control of a radiant panel heating system requires that the control device knows what is going on in each room and that it responds accordingly. There must be a direct relationship between what you sense to determine a call for heat and what you control. This is why I strongly recommend a multi-zone manifold control with multiple thermostats that read a call for heat in each zone and respond accordingly by opening the zone valve. Therein, you supply exactly the correct amount of heat energy where it is needed and when it is needed there, achieving true comfort and efficiency.
If someone installs a radiant panel heating system and opts to run the system with simple balancing manifolds and one thermostat, they might just as well install forced air heating, in my book. They will be effectively “throwing the baby out with the bathwater”.
Next, I will discuss the myths about outdoor reset and the flow vs. temperature control arguments.